The present invention relates to an electronic surveillance device making it possible in a predetermined space to detect and signal the presence or the absence of a body sufficiently conductive of hertzian-waves with the possibility of following its progress in the space to be controlled.
An electronic apparatus for detecting and signaling the presence of an object in a predetermined area is disclosed in U.S. Pat. No. 4,010,459. The apparatus, described in this patent, includes a micro-wave oscillator controlled by a modulator in order to obtain a hertzian-wave signal of 10 gigahertz modulated at 10 to 100 kilohertz. The signal is transmitted by a directional cornet horn type antenna. That apparatus is adapted to detect mobile material objects in industrial setting, but it is not adapted to discreetly detect a human intruder, taking into account that micro-waves signals are dangerous to the human body and that the predetermined space to control cannot be entirely covered by the radiation of the antenna, and that the antenna is too voluminous to be efficiently hidden and that, conventionally, the modulation signal is a constant low frequency triangular or square wave.
Another intrusion detector is disclosed in U.S. Pat. No. 4,605,922, the apparatus, described in this patent, includes a transmitter having a clock operating at a selected frequency, a pseudorandom code sequence generator for generating one of a selected number of codes, a modulator for spread spectrum modulating an electromagnetic signal with the generated pseudorandom code sequence signal and an antenna for transmitting the modulated signal through a zone of protection, and a receiver having an antenna, a demodulator for detecting and demodulating the signal, a clock operating at a frequency corresponding to the transmitter clock, a detector for comparing the received code signal, a phase detector, a control for phase synchronizing the two code signals, and a monitor for detecting and signalling changes in the detection signal resulting from entry or movement of an intruder in the zone of protection. That apparatus is adapted to detect mobile intruders only and it uses a micro-wave source and parabolic antenna, which have the inconveniences already cited above regarding discreet detection of persons.